The gas flow downstream of the impeller of an axial fan normally rotates. The rotational energy can be translated into useful energy by a guide vane arrangement on the outlet side of the fan, this arrangement converting the rotational velocity to an axial velocity component. The pressure is raised in this way, and the efficiency of the fan increases. Normally, however, the sound level also increases at the same time.
The sound from a fan comprises tonal components, i.e. tones with discrete frequencies and a wide band noise with a continuous frequency spectrum. Considerable efforts have been made primarily to lower the tonal components, by a suitable arrangement and embodiment of guide vanes on the output side of the fan.
It is accordingly a general understanding that the noise generated decreases with increasing distance between the impeller and guide vanes, see M. J. Benzakein, J. Acoust. Soc. Am. 51 (1972), 1427-1438 and W. Neise, Proc. INTER-NOISE 1988, pp 767-776. It has been found, however, this is not always applicable.
A guide vane is described in the Swedish patent 8802136-5, which has improved aerodynamic and acoustic properties.
It has also been found earlier that a non-uniform distribution of the guide vanes in the ring of guide vanes can give rise to certain acoustic improvements, although it has been found that large deviations from a uniform distribution of the guide vanes give rise to aerodynamic problems.
It is also known that certain acoustic characteristics can be improved by a portion being cut out from alternate guide vanes in their the forward portions.
FIG. 1 illustrates how the strength in an individual tone can be reduced by displacing the guide vanes in the circumferential direction of the guide vane ring. It will also be seen from the same figure that the noise at higher frequencies over about 500 Hz also increases at the same time. The measurement has been made for a fan R.P.M. of n=970 and a displacement of alternate guide vanes of .beta.=10.degree. and .beta.=0.degree..